Among color display devices used for image display such as computers and televisions, certain display devices equipped with plasma display panels (hereinafter referred to as “PDP” or “panel”) are drawing attention in recent years as color display devices for providing large screens while maintaining thin forms and light weights.
PDP presents full color display through the process of mixing the so-called three primary colors (i.e., red, green and blue). In order to make full color display, the PDP is provided with phosphor layers for luminous emission of the three individual primary colors of red (R), green (G) and blue (B), wherein phosphor particles composing the phosphor layers produce visible light of the individual colors when excited by ultraviolet rays generated inside discharge cells of the PDP.
Substances such as (Y, Gd) BO3:Eu3+ and Y2O3:Eu3+ which emit red light and become charged positively (+), Zn2SiO4:Mn2+ which emits green light and becomes charged negatively (−), BaMgAl10O17:Eu2+ which emits blue light and becomes charged positively (+), and the like chemical compounds are used as phosphors of the individual colors, as they are disclosed in “O plus E” (February, 1996, No. 195, pp 99-100) and the like non-patent publications, for instance.
In addition, “Phosphor Handbook” (Ohmsha, Ltd., pages 219-220) and other Non-patent publications disclose a technique in which each of these phosphors is made by the process of mixing certain raw materials and firing them at a temperature of 1000 deg-C. or higher to cause a solid phase reaction.
In a combination of the conventional phosphors of red, green and blue colors, only the green phosphor is charged negative, and this tends to cause discharge failures because an amount of electric charge carried in the green phosphor differs greatly as compared to those of the red and blue colors. Therefore, another technique is disclosed in Japanese Patent Unexamined Publication, No. 2001-236893, in which YBO3:Tb having positive charge (+) is mixed with Zn2 SiO4:Mn to bring the amount of electric charge as close as possible to those of the red and blue colors, and to avoid discharge failures. There is also another technique disclosed in Japanese Patent Unexamined Publication, No. 2003-7215, in which a discharging characteristic and brightness degradation are improved by using a mixture of BaAl12O19:Mn or BaMgAl14O23:Mn having a positive charge (+) and (Y, Gd) BO3:Tb or LaPO4:Tb, also having a positive charge (+).
However, there exists a problem with the green phosphor as described below, when producing a PDP of high brightness by increasing a density of Xe gas inside the PDP with any mixture of the prior art phosphor materials.
A panel made of BaMgAl10O17:Eu for the blue color, Zn2SiO4:Mn for the green color and a mixture of (Y, Gd) BO3:Eu and Y2O3:Eu for the red color carries positive charges (+) on the surfaces of the blue phosphor and the red phosphor among all these phosphors. However, since the green phosphor made of Zn2 SiO4:Mn has a larger ratio of SiO2 in proportion to ZnO (i.e., 1.5.ZnO/SiO2) than a stoichiometric ratio (2.ZnO/SiO2) for the reason attributed to manufacturing of the phosphor, a surface of Zn2SiO4:Mn crystal is covered with SiO2, and the phosphor surface is thus charged negative (−). In the PDP having both the phosphors charged negative (−) and positive (+) in coexistence, there generally is a problem that the negative charge remains only on the negatively charged phosphor, especially when the entire screen is put out after having been lit into a full luminance while the panel is driven, and this causes discharge failures such as instability and absence of discharge when the voltage is impressed for the subsequent image display. It has been found that these problems become exceptionally noticeable when the amount of Xe in the discharge gases is increased to 5% or more in an attempt to improve the brightness and efficiency of the PDP.
Furthermore, Zn2SiO4:Mn used for the green color remains in such a state that it is very liable to adsorb gases, especially because its surface is covered with SiO2. In other words, Zn2SiO4:Mn has adsorbed a large amount of moisture (H2O), carbon monoxide (CO), carbon dioxide (CO2), and/or hydrocarbon gases (CxHy) left as byproducts of the decomposed organic binder. They are gasified and released inside the panel during the aging process after the panel is sealed, and they result in degradation of the discharging characteristic when they are adsorbed in the surface of MgO. Moreover, these gases are adsorbed in the surfaces of the blue color phosphor of BaMgAl10O17:Eu and the green color phosphor of Zn2SiO4:Mn, and cause surface-reactions. As a result, there is a problem that brightness decreases and a y-value of chromaticity rises in the blue color, thereby decreasing a color temperature and causing blur in color of the panel.
On the other hand, there is a panel so contrived as to have a combination of green phosphors composed of Zn2SiO4:Mn having a surface of negative charge and ReBO3:Tb (“Re” represents a rare earth element such as Sc, Y, La, Ce and Gd) having a surface of positive charge, which are mixed in a manner that they are charged virtually positive (+), blue phosphor of BaMgAl10O17:Eu and red phosphor of (Y, Gd) BO3:Eu, both having positive charges. In the panel of this example, although there is an improvement to some extent of the discharge failures due to unbalance of the electric charges, it still increases the discharge failures when the density of Xe gas is raised.
In addition, this example causes deterioration of MgO due to H2O, CO, CO2 and CxHy gases released inside the panel during discharging operation as described above, and lowers the discharging characteristic including variations and failures of the discharge, since it contains Zn2SiO4 which is liable to adsorb H2O and CxHy gases. It also has a problem of degrading the brightness and causing blur in the color attributable to surface reaction of these gases with the phosphors of BaMgAl10O17:Eu and Zn2 SiO4:Mn.
The discharge failures can be prevented to some extent if all the phosphors are composed of the materials having positive charges (+), such that the panel employs a combination of green phosphor made of a mixture of any of BaAl12O19:Mn, BaMgAl10O23:Mn, (Y, Gd) BO3:Tb and LaPO4:Tb, all having positive charges (+), in place of the Zn2 SiO4 of negative charge (−), blue phosphor of BaMgAl10O17:Eu, and red phosphor of any of (Y, Gd) BO3:Eu and Y2O3:Eu.
However, this gives rise to another problem that failures and variations of discharge increase since the discharge voltage rises if the amount of Xe in the discharge gases exceeds 5% (or, especially when it exceeds 10%). Besides the degradation of these discharge characteristics, BaAl12O19:Mn and BaMgAl14O23:Mn, in particular, contain many defects within their crystal structures among the above green phosphors, and they are therefore liable to adsorb H2O and CxHy gases. In addition, LaPO4:Tb is also liable to adsorb H2O and hydrocarbon (CxHy) gases because it contains PO4 in the crystal structure.
For this reason, H2O and CxHy gases are released inside the panel during the aging process, and these gases cause chemical reactions over the surfaces of the phosphors, thereby accelerating degradation of the brightness during an extended period of lighting operation of the panel. Degradation of the brightness in the blue and green colors lowers a color temperature when the panel is lit for the full-on white screen, and this gives rise to still another problem in which the color of the panel blurs so that the screen becomes yellowish.
The present invention was contrived in consideration of the above problems, and it is an object of this invention to provide a plasma display device comprised of a green phosphor which is charged entirely with a positive potential (+), lessens adsorption of H2O, CO, CO2 and CxHy gases, and reduces chemical reactions to them.